Coil system



Patented Nov. 30, 1926.

UNITED STATES LESTER I4. JONES, OF ORADELL, NEW JERSEY.

COIL SYSTEM.

Applicationfiled May 22,

This invention relates to an inductance coil system and relates moreparticularly to a coil system especially designed for radio circuits ofthe sharply tuned or selective type; and has special reference to theprovision of a coil system which is self-shielded magnetically andelectrostatically.

As is well known, it is a desideratum in certain types of selective andsensitive radio receiving apparatus and in apparatus employable neartransmitting stations to construct the inductance coils of the receivingapparatus so that they are uncoupled magnetically and electrostaticallyto other coils and apparatus in the receiving circuit or to thetransmitting apparatus. To accomplish the magnetic uncoupling of thecoils to other apparatus, various types of coil windings have hithertobeen designed, such as double D windings, toroid windings and binocularcoils; and to accomplish the electrostatic uncoupling of such coils tosurrounding apparatus, it has been common to employ separateelectrostatic shields. These prior types of coil windings have, however,been found insufficient to produce the desired result-s on account oftheir relatively large external fields near the coil which produceconsiderable coupling to other parts and even between similar coils whenplaced at a moderate distance apart. To reduce the intermagneticcouplings between similar coils in a radio receiving set with the use ofsuch prior structures, it has been found necessary to so relativelyarrange the coils one with respect to the other in the radio receivingset as to minimize the reacting fields. Other structural difficulties,such as the large volume of winding required and the difficulty ofwindinc;- methods, have also hindered the use of these prior and knowntypes of winding coils.

A prime desideratum of my present invention resides in the provision ofan inductance coil system designed and constructed so that the externalmagnetic field is reduced to such a minimum that the coil issubstantially decoupled magnetically from surrounding apparatus and maybe placed in close proximity to other similar coils of a radioreceivingsystem without intermag- Y netically reacting with the same.

A further prime desideratum of the invention resides in the provision ofa coil system whlch is self-electrostatically shield- 1926. Serial No.111,074.

ed, that is, which requires no additional or separate electrostaticshield structure, the coil system being so designed and constructed thateven when employed in a sharply tuned circuit, the same will be soperfectly shielded electrostatically that the hand of the operator maybebrought to touch and surround the coil structure without detuning thecircuit.

It is a further principal object of my present invention to so designand construct the inductance coil system that the mathematical andempirical conditions which sat isfy the relations to produce themagnetic shielding also satisfy the relations to produce the staticshielding so that the coil system is both electromagnetically andelectrostatically decoupled from surrounding ap paratus, permitting notonly the close arrangement of a plurality of coil structures to produceacompactly organized radio receiving set, but permitting the same to beconstructed Without the use of electrostatic shielding devices.

To the accomplishment of the foregoing and such other objects as willhereinafter appear, my invention consists in the elements and theirrelation one to the other as hereinafter more particularly described andsought to be defined in the claims; reference being had to theaccompanying draws ings which show the preferred embodiment of myinvention, and in which:

Fig. 1 is a cross-sectional view of the coil system of my invention, and

Fig. 2 is a view of the same showing the relation between the magneticfield of the unshielded inner coil and the residual magnetic field ofthe coil system as a whole.

Referring now in detail to the drawings, and having reference first toFig. 1 thereof, the coil system of my invention is shown to comprise aninner coil section A and an out ter coil section B both preferably madein the form of solenoids arranged in coaxial relation, the inner coilsection being wound on a drum 10 made of insulating material and theouter coil section being wound on a drum 11 made of insulating material,the said'coil sections being related in the manner specified hereinafterso that the outer coil section B forms a magnetic and electrostaticshield for the inner coil section A.

The coil. sections'A and B are connected together at contiguous ends bythe conductor 12 so that the instantaneous currents traversing the coilsections are in opposite directions, or in other words, so that opposingfluxes of different magnitudes are produced in the core of the innercoil section A. Preferably as shown the outer coil joins the inner coilat the same axial end so that the two coils may be wound in the samedirection. The free end 13 of the inner coil section A is the highpotential end and is connected to the high potential point of the radioreceiving circuit, and the free end 14 of the outer coil section is theground end and is connected. in use to a point of ground potential inthe radio receiving circuit. For reasons to be pointed out hereinafter,the outer coil preferably overlaps the inner coil and preferably so thatthe line joining their ends makes an angle of approximately 45 with theaxis of the coils, as clearly shown in Fig. 1 of the drawings.

To produce the magnetic decoupling of v the-coil system, I havediscovered that the ratio of the turns of the two coil sections mustbear a definite and quite accurate rela tion to the ratio of the areasof the two coils. This relation is that the product ofthe area and thenumber. of turns on one coil section must be substantially equal'to theproduct- 'of the area and the number of turns of the other'coil section.The satisfaction of this condition reduces to a minimum the magneticcoupling between the coil system and a uniform magnetic field, as wellas the magnetic coupling of the coil system with other similar coils.This will be best understood when it is seen that'if the coil system ofmy invention be placed in a uniform magnetic field of variableintensity, which field is directed for point of illustration along theaxes of the coils, the E. M. F. induced in the inner coil section isequal. to the E. M. F.

induced in the outer coil section, since the induced E. M. F. in a coilsection is a linear function of the number of turns and the total fluxthreading the coil. Since the product of the area and number of turnsfor one coil is equal to the product of the number of turns and the areafor the other coil, it follows that the induced E. M. F.s are equal.Conversely, it will therefore be seen that the resultant field at adistance is zero, since the field produced by one coil sectionneutralizes at a distance the field produced by the other coil section.If the field be not parallel to the axis as assumed. the samel'conditionexists, since the field threading the coil is to be taken as the productofthe field intensity and the cosine of the angle between the coil axisand the direction of the field, this angle being the same for both coilsections. a To produce the static decoupling or shielding of the coilsystem. I have discovered that the coil system should be so constructedthat the self-inductanceof the, outer coil section B be madesubstantially equal to the mutual inductance between the two coilsections A and B. When this condition is satisfied, there results thenovel effect of no potential drop along the outer coil due to the highfrequency currents flowing; so that the whole surface of the outer coilis at substantially one potential. If this potential be made the groundpotential as shown in the preferred use of the coil system, it will beseen that the coil system is substantially completely electrostaticallyshielded.

To make the self-inductance of the outer coil section 13 equal to themutual inductance between the coils, I have found first that the ratioof the turns of the two coils must bear the same relation to the ratioof the area of the two coils as is necessary to satisfy the requirementof producing. the magnetic shielding of the coil system hereinbeforedescribed. Thus the product of the area and number of turns of one coilshould be equal to the product of the area and the number of turns ofthe other coil. satisfying this condition, which is the same forproducing magnetic decoupling, it is generally necessary only to makethe length of the outer coil slightly greater-than the length of theinner coil, as clearly shown in the drawings. This difference in coillength may be utilized as shown to improve the electrostatic shieldingof the high potential end of the inner coil, or, where this is not ofprimary importance, the inner and outer coils may be relatedsymmetrically with a little overhanging of the outer coil at each end.The latter slightly impairs the electrostatic shielding of the highpotential end of the inner coil but slightly improves the magneticshielding of the coil system.

\Vhen exactcompliance with the lastnientioned condition is effected, Ihave found that it is possible to place ones hands about the outer coilwithout detuning, even though this coil be connected in a sharply tunedcircuit.

To produce a coil system of high efiiciency, I have found that the ratioof areas of the coil sections should bear a given relation. Generallythe ratio of areas of the outer to the inner coil section may be between5 to 1 and 1 to 1, and for producing the greatest. efficiency the ratioof areas should be about 2, 2.1 and 2.2 for coils having ratios of 1.26,1.58 and 2.1 respectively, of outer coil diam eter to, length. Thesearea ratios are considered desirable because they give a maximuminductance for a given length of coil system having a constant outsidediameter when the inner coil is wound with a constant number of turns.This is a practical commercial limitation becauseother electrical andstructural conditions impose certain limitations on coil diameters andcoil lengths. The maximum number of turns per inch that After.

lilil Magnetically considered, the magnetic flux can be wound on thesolenoid form is usually determined by the wire diameter especially atshort waves where multilayer or banked windings cause a large decreasein coil efiiciency. Where the area ratio of outer coil to inner coil isabout 2 I also obtain a high degree of static shielding with lowdistributed capacity in addition to obtaining maximum inductance withminimum resistance.

The results of magnetic and electrostatic shielding produced by my coilconstruction may also be visualized by consideration of the magnetic andelectrostatic fields pro.- duced by the inter-acting coil sections.

through the inner coil due to the current in the inner coil, which fluxis the more intense flux, is slightly reduced by the opposing fluxthrough the inner coil due to the current in the outer coil. Theinductance of the coil system arises primarily because" the inner coilsection has in its core a rela- -sbf 175at a-point along the axis of thecoil, partially neutralized by the flux of theouter coil. This partiallyneutralized flux witlziin,

tivelyintense magnetic field which is only the coil has for its returnpath the annular space between the coils. That portion of the flux ofthe inner coil which would tend {30 return outside the outer coil isneutralize follows, therefore, that the outer coil section confines themagnetic flux and is effective as a magnetic shield. I

Electrostatically considered, we may first assume that we may neglectthe potential drop along the outer coil section due to the resistance ofthe coil, since for the sharply tuned circuits in which these coils areto be used, the ratio of inductive to resistive drop is greater thanto 1. Now the inductive drop along the outer coilmust be due to themagnetic flux linkages with this coil. Sinc practically the entire fluxthreading the coil system passes through the core of the inner.

coil and back in the annular space betweenthe coils, there is noresultant flux linkage with the outer coil. This statement is to bemodified only with respect to the residual flux which I have foundissues radially from the center of the coil system and tends to produceneutralizing flux linkages in the hal es of the outer coil asillustrated in Fig. 2 of the drawings, which will be referred tohereinafter. Looking at the phenomenon from the viewpoint of self andmutual inductances, the inductive drop along the return flux of theouter coil; It

much greater than the self-inductance of the outer coil, the fraction ofinductance of the inner coil lost due to the reverse mutual isinconsequential.

The manner of designing and constructing the coil system of my presentinvention will in the main be fully apparent from the above detaileddescription thereof. The uses and the manifold advantages, especially inradio frequency circuits of the tuned and selective types, will, it isthought, be also manifest from the above. To show the substantialelimination of the magnetic field surrounding the coil system, Ireproduce graph) the small magnetic residual field of the coil system asa whole. Thus it will be-seen, viewing Fig. 2 of the drawings, thatWhile the inner coil (unshielded) has a value the,.value of the residualwhen the coil is 'cbn'nected to its. outer shielding section falls atthe 'i'same point to a value too small for ,measiire'ment. Comparingpoints along a centralgline normal to the axis of the coil,

he field of the unshielded coil at a point rom the coil :center equal tofour times the "-radiusz'f'of the coil .is 104, while a similar point:of the shielded coil has a recorded value of G. v

= While I have-shown and described my invention in the preferred form,it will be apparent that many changes and modifications may be made inthe structure disclosed without departing from the spirit of theinvention,?defined in the following claims.

'I-claim:

' 1. A coil system comprising an inner coil section and an outer soilsection connected to produce opposing fluxes of different magnitudes inthe core of the inner coil section, the product of the number of turnsand area of the inner coil section being related to the product of thenumber of turns and area of the outer coil section to produceneutralizing magnetic fields at a distance.

2.;A coil system comprising inner and outer coa'xially arrangedcylindrical coil sections connected to produce opposing fluxes ofdifferent magnitudes in the core of the inner coil section and a flux inthe annular space between the coil sections, the product of the numberof turns and area of the inner the outer coil due to its self-inductanceis coil section being related to the product of neutralized by thevoltage drop due to the induced voltage arising from the mutualinductance of the two coils, these being equalized. Similarly thevoltage drop along the inner coil is neutralized in part by the mutualinductance between the coils, but

the self-inductance of the inner coil being the number of turns and areaof the outer coil section to produce neutralizing magnetic fields at adistance.

3. A coil 5 stem comprising an inner inductance coi section and an outershielding coil section connected so that the instantaneous currentstraversing the same are in tance coil section being equal to the productof the number of turns and area of the out 5 er shielding coil section.

v 4. Aj'co'il system comprising inner and outer coairially arrangedcylindrical coil unctions connected so that the instantaneous currentstraversing the same are in opposite directions producing opposing fluxesof different magnitudes in the core of the inner coil section and a fluxin the annular space between the coil sections, the product of thenumber of turns and the area of the inner e il section being equal tothe product of e'number of turns and area of the outer coil section.

5. A coil system comprising an inner coil section and an outer coilsection connected so that the instantaneous currents traversing the sameare in opposite directions, the ratio of areas of the outer to the innercoil sections being between five to one and one and one-half to' one,and the product of the number of turns and the area of the inner coilsection being equal to the product of the number of turns and area ofthe outer coil section.

6. A coil system comprising an inner coil sect-ion and an outer coilsection connected so that the instantaneous currents traversing the sameare in opposite directions, the ratio of areas of the outer to the innercoil sections being substantially two to one, and

the product of the number of turns and the area of the inner coilsection being equal to the product of the number of turns and area ofthe outer coil section.

7. A coil system comprising an inner coil section and an outer coilsection connected to produce opposing fluxes of different magnitudes inthe core of the inner coil section, the self-inductance of the outercoil section being equal to the mutual inductance be tween the coilsections, whereby the outer coil section forms a static shield for thecoil system.

8. A coil system comprising inner and outer coaxially arrangedcylindrical coil sections connected to produce opposing fluxes ofdifferent magnitudes in the core of the inner coil and a flux in theannular s ace between the coils, the self-inductance o the outercoilsection being equal to the mutual inductance between the coil sectionswhereby the outer coil section forms a static shield for the coilsystem.

9. A coil system comprising inner and outer coaxially arrangedcylindrical coil sections connected to produce opposing fluxes ofdifferent magnitudes in the core of the inner coil and'a flux in theannular space nected to produce opposing fluxes of differ.

ent magnitudes in the core of the inner coil section, the product of thenumber of turns and area of the inner coil section being related to theproduct of the number of turns and area of the outer coil section toproduce neutralizing magnetic fields at a distance 7 and so that theself-inductance of the outer coil section is equal to the mutual inductan e between the coil sections.

11. A coil system comprising an inductance coil section and a shieldingcoil section connected thereto, the area turns of the coil sectionsbeing related to produce neutralizing magnetic fields at a distance andso that the self-inductance of the shielding coil section equals themutual inductance between the coil sections, whereby the shielding coilsection forms a static and magnetic shield for the inductance coilsection.

12. A coil system comprising an inner inductance coil section and anouter shielding co'l section connected so that the instantaneouscurrents traversing the same are in opposite directions, the product ofthe number of turns and area of the inner inductance coil section beingequal to the product of the number of turns and area of the outershielding coil section, and the length of the outer coil section beingslightly greater than the length of the inner coil section whereby theouter coil section forms the static and magnetic shield for the coilsystem.

13. A coil system comprising an inner coil section and an outer coilsection connected so that the instantaneous currents traversing thesameare in opposite directions, the ratio of areas of the outer to the innercoil sections being substantially two to one, and the product of thenumber of turns and the area of the inner coil section being equal tothe product of the number of turns and area of the outer coil section,

/ and the length of the outer coil section being LESTER L. JONES.

